Method for making high flow porous reverse osmosis membranes containing lipids

ABSTRACT

A SEMI-PERMEABLE MEMBRANE CONTAINING A SUPPORTIVE LAYER AND A LIPID CONTAINING OSMOTIC SKIN LAYER OF POLYMERIC FILM FORMING CELLULOSIC MATERIAL IS MADE BY (1) ADMIXING FILM FORMING CELLULOSIC MATERIAL, SOLVENT, SWELLING ADDITIVE AND LIPID TO PROVIDE A CASTING SOLUTION (2) CASTING A FILM FROM THE SOLUTION (3) DRYING THE FILM (4) LEACHING THE FILM TO FORM A SEMI-PERMEABLE OSMOTIC SKIN MEMBRANE CONTAINING A RESIDUE OF LIPID, AND (5) OPTIONALLY CURING THE MEMBRANE.

Dec. 18, 1973 sTANA 3,780,147

METHOD FOR MAKlNG HIGH FLOW POROUS REVERSE OSMOSIS MEMBRANES CONTAINING IJTPI13S Original Filed May 13, 1969 3 Sheets-Sheet 1 2 1 3 SEA WATER A.

OR A A g 0 F POWER RECOVERY CONTAMINATEDWATER A 0/ M TURBINE o o d 0 4M T 7/ CASTING SOLUTION FIG. 3

.R. R. STANA Dec. 18, 1973 S I S O M S O E S R MS Em R P T S L w G R ow PEN WM T L N 0 C H 3 I.) M n N B I m M M H U If D O H T E M 3 Sheets-Sheet 2 Original Filed May 13,

m w NH E 0 LM R W w T 3 W m M M N m U F [L R U C O N D F ll o P 4 Cll 7 11 F NO 0 N 0 0 0/\ 00 K 1 0 0 0 0 0 0 0 0 zoCQwEm QB 2 0 FLUX (gal'lft day) D E E R U C .T m E .A R 6 D U E C G N F F U 0 0 .l. E E N E U 0 O 0 0 0 0 0 00 7 6 5 4 FIG. 6B

LINE E 160F CURE AIR DRY TIME SEC Dc. 18, 1973 s A 3,780,147

METHOD FOR MAKING HIGH FLOW POROUS REVERSE OSMOSIS MEMBRANES CONTAINING LTPIDS POROUS CELLUOSE ACETATE SUPPORTIVE LAYER United States Patent ABSTRACT OF THE DISCLOSURE A semi-permeable membrane containing a supportive layer and a lipid containing osmotic skin layer of polymeric film forming cellulosic material is made by (l) admixing film forming cellulosic material, solvent, swelling additive and lipid to provide a casting solution (2) casting a film from the solution (3) drying the film (4) leaching the film to form a semi-permeable osmotic skin membrane containing a residue of lipid, and (5) optionally curing the membrane.

CROSS REFERENCE TO RELATED APPLICATIONS The present application is a divisional application of application U.S. Ser. No. 824,115, filed on May 13, 1969 now Pat. No. 3,593,855.

BACKGROUND OF THE INVENTION The reverse osmosis method of water purification, employing cellulose acetate membranes, ofiers promise in solving the problem of purifying waste waters economically. The issues of water shortage and pollution are of growing concern in the United States and throughout the world. The next few decades may well see the demand for water in the United States exceed its supply. Therefore, there is a pressing need for the commercialization of an economic method of water purification to supply new population and industry demands.

The investigations of Breton and Reid at the University of Florida revealed that cellulose acetate was an excellent film forming material for semi-permeable membranes. Loeb, at the University of California modified the preparation of these membranes by incorporating aqueous magnesium perchlorate in an acetone solution of cellulose acetate.

These standard membrances will only reject to 30 percent of the dissolved solids in water fed through a reverse osmosis system, but this may be improved by curing the membranes in a hot water bath at temperatures up to 190 degrees F. The higher curing temperature gives the higher rejections; however, the higher the rejection, the lower the flux of purified product through the membrane. A membrane which gives 30 gal/sq. ft.- day at 20 percent rejection will only give 1-2 gal/sq. ft.-day at 95 percent rejection of impurities.

This low flux at high rejection is one reason why reverse osmosis has not yet become a full commercial development. The problems of increased purified water flux at high rejection rates for cured membranes and improving rejection and flux for uncured membranes, so that it would be possible to use low temperature plastics for membrane cell supports, have heretofore been unsolved.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide improved reverse osmosis semi-permeable membranes, adapted to permit the rejection of dissolved impurities in a solution, having dense osmotic skins containing lipids for use in tubular or other type reverse osmosis system.

It is another object of this invention to provide a new and improved method for fabricating these semipermeable membrances wherein lipids are incorporated into the dense osmotic skin of the membrane.

Briefly, these objects are accomplished by (1) using glycerol ester lipids in a cellulose acetate, acetone, formamide casting solution within a critical weight percent range (2) casting this casting solution to form a film 3) air drying the cast film within a critical air dry time range (4) immersing the cast film in an ice water leaching bath to form a semi-permeable osmotic skin membrane and (5) optionally curing the membrane in a hot water bath up to about 200 degrees F.

My semi-permeable lipid containing membranes can be used for applications on tap water for use as water softener and taste improvement, sea water and brackish water for desalination applications, waste efiiuents for waste impurity disposal, and urea water for biological applications. I have found that my lipid containing mem branes exhibit up to two times the flux, at a given solute rejection and pressure, as standard unmodified cellulose acetate membranes. They also can give an increase in solute rejection by the initial uncured membrane.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the nature and objects of the invention, reference may be had to the following drawings in which:

FIG. 1 is a diagram showing the principle for extracting fresh water from sea or contaminated water utilizing a tubular reverse osmosis system;

FIG. 2 illustrates one theory of water effusion and rejection of solute impurities for a semi-permeable membrane containing a dense osmotic skin layer of polymeric film forming cellulosic material having a plurality of foraminous passageways in its structure and containing lipids in an amount to effect improvement in rejection of impurities;

FIG. 3 is a cross sectional illustration of a casting apparatus used in the experiments;

FIG. 4 is a cross sectional illustration of one of the membrane cells used in testing the semi-permeable membranes of this invention;

FIG. 5 is a graph comparing performance of semipermeable membranes containing lipids with those not containing lipids; and

FIG. 6 shows graphs of air dry time versus flux and air dry time versus percent rejection rates of a semipermeable membrance containing lipids.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Osmosis occurs when two solutions of different concentrations in the same solvent are separated from one another by a membrane. If the membrane is ideally semipermeable, that is, if it is permeable to the solvent and not to the solute, then a flow of solvent occurs from the more dilute into the more concentrated solution. This continues until the two solutions become equal in concentration or until the pressure in the chamber of the more concentrated solution rises to a certain well-defined value. The pressure difference at which no flow occurs is termed the osmotic pressure difference between the two solutions. If a pressure in excess of this osmotic pressure difference is applied to the more concentrated solution, then the solvent can be caused to flow intothe more dilute solution. The names reverse osmosis, pressure osmosis and hyperfiltration are used to describe this process. A detailed analysis of this phenomenon can be found in U. Merten, Desalination by Reverse Osmosis, M.I.T. Press, 1966.

FIG. 1 illustrates a typical tubular type reverse osmosis system. Sea water or contaminated water is pumped through a battery of support tubes 1. The pump 2 must exert a pressure greater than the osmotic pressure of the feed and can operate as high as 4000 p.s.i. The tubes can be mounted in batteries of about 150 each. They serve to support the reverse osmosis membrane contained within the tube wall.

The tube walls that support the reverse osmosis membranes must be able to withstand the pressure exerted on them by the pump and must be able to allow egress of the pure water 3 into a collecting pan 4. For sea water several stages of this type may be required before the water is usable.

Suitable support modules both for tubular and fiat plate reverse osmosis systems are well known and can be used with the semi-permeable lipid containing reverse osmosis membranes of my invention.

An interesting peculiarity of semi-permeable cellulose acetate reverse osmosis membranes is that they will only reject salt or other solute impurities if one particular side is in contact with the contaminated water or other feed. The cellulose acetate osmotic skin layer of the membrane is extremely thin, about 0.2 micron thick and has minute foraminous passageways through it as shown schematically in FIG. 2 of the drawings. It is very dense and is the side that is effective in holding back the dissolved impurities in the feed. The remainder of the membrane structure contains a spongy porous cellulose acetate sublayer material that accounts for about 99.8 percent of the membranes thickness. Its function is mainly supportive.

The process by which the dense osmotic skin of cellulose acetate does its job is complex. One theory is that the polymer has strong hydrogen bonding groups that enhance the bonding of water. The molecules of water migrate from one hydrogen bond site to the next within this dense osmotic skin layer through the minute foraminous passageways. After this effusion through the active osmotic skin layer, they recombine and flow through much more porous cellulose acetate supportive layer as shown in FIG. 2 of the drawings.

The exact mechanism by which the lipids (mixtures of saturated and unsaturated esters of glycerol) affect the membrane characteristics is not precisely known at this time. The lipids are surface active with respect to the water-membrane interface and thus, even though they are generally insoluble in water, they are attracted to the water interface. From physical examination of the cast semi-permeable membranes that contain lipids, it is evident that the lipids are concentrated on the dense osmotic skin side. Membranes cast from solutions not containing a lipid appear about the same on either side whereas membranes containing a lipid appear glossy on the osmotic skin side. This is particularly apparent in the dried membranes where the osmotic skin side has a slippery oily feeling.

It is thought that rejection of dissolved impurities in the water or other feed is improved, with the use of effective amounts of these charged lipid groups, by their concentration at the surface of the osmotic skin and in the plurality of foraminous water passages through the osmotic membrane skin as shown in FIG. 2. It is thought that the lipids line the surface and passages through the osmotic skin, lining up in an electrical double layer at the water-membrane interface and giving the lining of the minute passageways a more negative charge. This negative charge allows effusion of neutrally charged molecules through the membrane while rejecting charged ions. Since these lipid molecules can be fairly small compared to cellulose acetate, they are fairly mobile and as a result only a small concentration may be needed to cover all the water interfaces. Although the exact mechanism is not precisely known, it is known that effective amounts of lipid incorporated into the osmotic skin layer give superior membranes.

In the method of my invention a polymeric film forming cellulosic material such as a cellulosic ether or ester derivative is dissolved in a volatile solution, containing lipids, solvents and swelling additives, capable of causng the structural organization of a dense osmotic membrane skin from the cellulosic ether or ester. Preferably the solvent is acetone, the swelling additive is formamide and the cellulosic derivative is cellulose acetate. The lipid must constitute at least 0.1 wt. percent of the casting solution. The casting solution is then cast as a film in fiat or cylindrical form and a portion of the casting solution is allowed to evaporate during an air dry time ranging from 0 to about seconds, causing a dense osmotic skin to develop at the air-solution interface. The cast film is then immersed in a leaching bath containing a leaching solvent such as water where the organic solvent and swelling agent pass through the osmotic skin into the leaching bath and the water from the bath passes into the membrane. The semi-permeable membrane may then be cured causing the cellulose acetate to shrink, presumably reducing the size of the foraminous passageways through the osmotic skin and allowing the molecules to align themselves in a regular pattern so as to have all the negative groups lining the passageways. Feed water containing dissolved solids is applied to the high density lipid containing osmotic skin side of the semi-permeable membrane under pressure and purified water is recovered from the opposite side.

It is also possible to incorporate the necessary lipid into the semi-permeable membrane in an amount to effect improvement in rejection of impurities and the effusion of water through the membrane by first casting a suitable membrane from a casting solution not containing any lipid constituent, and then applying or incorporating the lipid to the osmotic skin of the membrane, as for example, by painting on a lipid solution.

The polymeric film-forming cellulosic ether or ester matertial derivative that can be used in the method of this invention can have the general formula:

wherein R R and R are members of a group consistmg of H, R and CR O where H is hydrogen, R is an alkyl group containing 1 to 8 carbon atoms and R is an alkyl group containing 1 to 7 carbon atoms. Specific examples of such polymeric film forming materials are cellulose acetate, cellulose acetate-butyrate, cellulose propionate and ethyl cellulose.

The preparation of such materials is well known in the art. Cellulose acetate, the preferred polymeric film-forming cellulosic material, can be prepared by first converting cellulose into the triacetate in the presence of acetic anhydride, acetic acid and sulfuric acid. Partial hydrolysis removes some of the acetate groups, degrades the chains to smaller fragments (of 200-300 units each) and yields cellulose acetate. Detailed description of preparation and structures of these cellulose ethers and cellulose esters can be found in Brydson, Plastics Materials, D. Van Nostrand, pp. 356-376 (1966).

The volatile casting solution used in the present invention contains solvents and pore producing swelling additives adapted to permit the structural organization of the osmotic skin from a film forming material by evaporation and leaching of the carrier solution. Preferably the casting solution contains a pore producing swelling additive such as formamide (methanamide) I-IC'ONH triethyl phosphate, tetrahydrofurfuryl phosphate, and an organic solvent, such as acetone, methyl ethyl ketone, ethyl alcohol or methyl alcohol.

The swelling additive is one which will not dissolve the cellulose acetate but is compatible with it. Others would include aqueous magnesium perchlorate and similar materials.

The lipids most useful in this invention are esters of carboxylic acids. These esters are derived from glycerol (HOCH CHOHCH OH) and hence are known as glycerides. The basic ,structure of these glycerol ester lipids is shown below:

wherein R, R, R" are each alkyl groups containing 1 to 20 carbons and preferably 1 to 8 carbons. These glycerol ester lipids are mixed glyceryl esters of various saturated and unsaturated, straight chain, even-numbered carbon acids. The most abundant saturated acids are the straight-chain even-numbered carbon acids:

caprylic (CH (CH COOH),

capric (CH (CH COOH) lauric (CH (CH COOH), myristic (CH (CH C0OH),

palmitic (CH (CH COOH), and

stearic (CH CH COOH) The common unsaturated acids are oleic (CH (CH CH=CH(CH COOH),

linolenic (CH3(CH2)4CH=CHCH2CH -=CH(CH2)7COOH),

and

linolenic CH CH CH=CHCH CH =CHCH CH=CH(CH )';COOH).

The major constituents of the glycerol ester lipids are the acids, which represent up to 90 percent of their weight.

Examples of the acid composition of some glycerol ester lipids are given in Table 1 and Table 2 below:

Triacetin. Olive oil Cottonseed oil.

The preferred glycerol ester lipid is triacetin ('triacetate glycerol):

HzC-O-(f-CH;

HC-O-C-CH;

because of its complete solubility in the casting solution. This lipid can be formed by the reaction of acetic acid and glycerol:

H IEhC-OH HzC-O-C-CHI 3 H--(f-OH] l- HK'J-OH iii (l H; OH HC-O-O-CH; 31310 HzC-O-C-CH;

(acetic acid) (glycerol) (a glycerol ester lipid) Further details on the composition and synthesis of glycerol ester lipids is readily available and can be found in Giiman, Organic Chemistry-An Advanced Treatise, John Wiley & Sons, vol. 3, Chapter 3, 1953, and Fieser and Fieser, Advanced Organic Chemistry, Reinhold Pub., Chapter 30, 1961, herein incorporated by reference. It was found that as little as 0.1 wt. percent lipid in the casting solution was effective in producing increased membrane rejection and although good membranes have been produced at about 4 wt. percent, improvement tapers 01f at concentrations above about 2.5 wt. percent.

The experimental equipment consisted of a film sheet casting apparatus, film tube casting apparatus, a leaching bath, a high pressure test loop for testing either tubular or flat membranes and a tap water test loop using the tap water line pressure, the film membrane casting. apparatus used in the examples is shown in FIG. 3 and consists of a hollow container 30 into which the casting solution is poured. One side of this container has a blade 31 which adjusts the height of a hole 32 extending along the length of the container. The casting solution flows through the hole and onto the glass plate 33, as the container is moved to form the cast film 34. The leaching bath consists of a bottom tank where an ice water mixture is kept, and a pair of top water trays to which the ice water from the bottom tank is constantly circulated. The film tube casting apparatus was designed to be able to cast four foot long tubes and at the same time be able to control the casting rate, membrane thickness, air dry time, flow of leaching water past the membrane and leaching time. The high pressure loop used a Mayno pump that could provide up to 2 gallons per minute at pressures up to 500 p.s.i.g. Feed water for this system consisted of 5000 p.p.m. sodium chloride solution. The tap water loop used laboratory tap water at 70-80 p.s.i.g. and 200 p.p.m. (parts per million) dissolved solids as a feed.

The membrane cells used for testing flat membranes were of the standard type and used Type 304 stainless steel or plexiglass as construction materials. Round cells were used for testing 1.0 inch diameter membranes. This cell, used in the examples, is shown in FIG. 4 of the drawings. The feed water side seal is made with a rubber O ring 40. The product side collection consisted of fine stainless steel wire cloth 41 covered with 2 layers of paper 42, with the paper contacting the cast semi-permeable membrane 43 having an osmotic skin 44. Feed water is fed in opening 45, which is angled to distribute the water, and out opening 46. Purified water is removed through opening 47.

7 EXAMPLE 1 A casting solution was prepared having the following composition:

9 oz. 43.2 wt. percent cellulose acetate 5 oz. 24.1 wt. percent acetone 6 oz. 28.9 wt. percent formamide (methanamide) 25 ml. 3.8 wt. percent olive oil (glycerol ester lipid having an acid composition of about palmitic and stearic acids, 83% oleic acid and 7% linoleic acid.)

The solid cellulose acetate was added to the other liquid ingredients to give a viscous liquid solution. This viscous casting solution was poured into the film casting apparatus, heretofore described and shown in FIG. 3 of the drawings, containing a blade with an opening of 12 mils. The casting apparatus was 1 ft. long and it was hand drawn across a 1 x 1' glass plate in 5 seconds to give a cast film about 10" x 10". The casting was allowed to air dry for another 55 seconds to given an air dry time of about 60 sec. Both the glass plate and the casting apparatus were at room temperature. No effort was made to increase the normal laboratory air circulation during the air dry time. The glass plate and casting were then immersed in a leaching bath prepared from a mixture of tap water and crushed ice. After 20 minutes the semipermeable membrane casting 12 mils thick was separated from the glass plate. The osmotic skin side appeared glossy. A small sample 1" diameter was cut out of the uncured semi-permeable membrane and mounted in a standard stainless membrane cell heretofore described and shown in FIG. 4 of the drawings.

Tap water containing 100 ppm. dissolved solids was fed into the membrane cell against the osmotic skin side of the membrane at 80 p.s.i. The uncured semi-permeable membrane operated at 86% rejection of dissolved solids with a purified water flux of 2.5 gaL/sq. ft. day.

EXAMPLE 2 A casting solution was prepared having the following composition:

5 oz. 25.7 wt. percent cellulose acetate 9 oz. 46.2 wt. percent acetone 5 oz. 25.7 wt. percent formamide (methanamide) 15 ml. 2.4 wt. percent cottonseed oil (glycerol ester lipid having an acid composition of about 27% palmitic and stearic acids, 19% oleic acid and 54% linoleic acid).

The solid cellulose acetate was added to the other liquid ingredients to give a viscous liquid solution. This viscous casting solution was poured into the film casting apparatus, heretofore described and shown in FIG. 3 of the drawings, containing a blade with an opening of 12 mils. The casting apparatus was 1 ft. long and it was hand drawn across a 1' x 1'. glass plate in 5 seconds to give a casting about 10" x 10". The casting was allowed to air dry for another 55 seconds to give an air dry time of about 60 seconds. Both the glass plate and the casting apparatus were at room temperature. No eifort was made to increase the normal laboratory air circulation during the air dry time. The glass plate and casting were then immersed in a leaching bath prepared from a mixture of tap water and crushed ice. After 20 minutes the semipermeable membrane casting 12 mils thick was separated from the glass plate. The osmotic skin side appeared glossy. A small sample of the uncured semi-permeable membrane 1" diameter was then cut out and set aside.

The remaining uncured semi-permeable membrane was immersed in a 154 degree F. water curing bath for 10 minutes. A small sample of the semi-permeable membrane 1" diameter was then cut out and set aside.

The remaining semi-permeable membrane was immersed in a 160 degree F. water curing bath for 10 minutes. A small sample of the semi-permeable membrane 1" diameter was then cut out and set aside.

These three semi-permeable membranes (uncured, 154 degree F. cured and 160 degree F. cured) were then mounted in the standard stainless membrane cells heretofore described and shown in FIG. 4 of the drawings.

Tap water containing 100 p.p.m. dissolved solids was fed into the membrane cells against the osmotic skin side of the membrane at p.s.i.

The uncured semi-permeable membrane operated at 62% rejection of dissolved solids with a purified water flux of 9.2 gal./sq. ft. day.

The 154 degree F. cured semi-permeable membrane operated at 78% rejection of dissolved solids with a purified water flux of 4.4 gaL/sq. ft. day.

The 160 degree F. cured semi-permeable membrane operated at rejection of dissolved solids with a purified water flux of 2.7 gaL/sq. ft. day.

EXAMPLE 3 Three casting solutions were prepared having the following compositions:

Solution A 4.5 oz. 21.8 wt. percent cellulose acetate 10.0 oz. 48.7 wt. percent acetone 6.0 oz. 29.2 wt. percent formamide (methanamide) 2 ml. .3 wt. percent triacetin (triacetate glycerol) Solution B 5 oz 23.6 wt. percent cellulose acetate 10 oz. 47.7 Wt. percent acetone 6 oz. 28.4 wt. percent formamide (methanamide) 5 ml. .6 wt. percent triacetin (triacetate glycerol) Solution C 5 oz. 25.0 wt. percent cellulose acetate 9 oz. 45.0 wt. percent acetone 6 oz. 30.0 wt. percent formamide (methanamide) This casting solution contained no lipid, but is regarded as an excellent high rejection casting solution for semipermeable reverse osmosis membranes.

This example will illustrate the superiority of (membranes containing lipids over membranes) not containing lipids but having comparable wt. percent starting ingredients, in tests under identical laboratory conditions.

The solid cellulose acetate was added in all three cases to the other liquid ingredients to give a viscous liquid solution. Each of these viscous casting solutions were poured into film casting apparatus, heretofore described and shown in FIG. 3 of the drawings, containing a blade with an opening of 6 mils. The casting apparatus was 1 foot long and in all three cases it was hand drawn across a 1' x 1' glass plate in 5 seconds to give three castings each about 10" x 12". All three castings were allowed to air dry for another 5 seconds to give an air dry time of about 10 seconds for each of the three castings. Then each glass plate and casting was immersed in a leaching bath prepared from a mixture of tap water and crushed ice. After 20 minutes the three semi-permeable membrane castings 6 mils thick were separated from their respective glass plates. The osmotic skin sides of the Solutions A and B membranes appeared glossy. A small sample of each of the uncured semi-permeable membranes, 1" diameter was then cutout and set aside.

Solution A membrane The remaining uncured semi-permeable membrane cast from Solution A was immersed in a degree F. water curing bath for 10 minutes. A small sample of the semipermeable membrane 1 diameter was then cut out and set aside.

The remaining semi-permeable membrane was immersed in a degree F. water curing bath for 10 minutes. A small sample of the semi-permeable membrane 1" diameter was then cut out and set aside.

The same procedure was followed to get samples of semi-permeable membranes cured at 180 degree F. and 190 degree F.

Solution B membrane The remaining uncured semi-permeable membrane cast from Solution B was immersed in a 160 degree F. water curing bath for minutes. A small sample of the semipermeable membrane 1" diameter was then cut out and set aside.

The remaining semi-permeable membrane was immersed in a 174 degree F. water curing bath for 10 minutes. A small sample of the semi-permeable membrane 1" diameter was then cut out and set aside.

The same procedure was followed to get samples of semi-permeable membranes cured at 180 degree F. and 190 degree F.

Solution C membrane The remaining uncured semi-permeable membrane cast from Solution C was immersed in a 160 degree F. water curing bath for 10 minutes. A small sample of the semipermeable'membrane 1" diameter was then cut out and set aside.

The remaining semi-permeable membrane was immersed in a 174 degree F. water curing bath for 10 minutes. A small sample of the semi-permeable membrane 1" diameter was then cut out and set aside.

The same procedure was followed to get samples of semi-permeable membranes cured at 180 degree F. and 190 degree F.

This gave five Solution A membrane samples (uncured, 145 degree F. cure, 170 degree F. cure, 180 degree F. cure and 190 degree F. cure) five Solution B membrane samples (uncured, 160 degree F. cure, 174 degree F. cure, 180 degree F. cure and 190 degree F. cure) and five Solution C membrane samples (uncured, 160 degree F. cure, 174 degree F. cure, 180 degree F. cure and 190 degree F. cure).

These fifteen semi-permeable reverse osmosis membranes were then mounted in the standard stainless membrane cells heretofore described and shown in FIG. 4 of the drawings. In all cases both the glass plate and the casting apparatus were at room temperature. No effort was made to increase the normal laboratory air circulation during the air dry time. In all cases the air dry time was about 10 seconds and the membrane thickness was 6 mils.

In all cases tap water containing 100 p.p.m. dissolved solids was fed into the respective membrane cells against the osmotic skin sides of the membranes at 80 p.s.i. The comparative results are shown in FIG. 5, a graph of purified water flux vs. percent rejection of dissolved solids in the tap water feed. Line A represents the performance of the semi-permeable membranes cast from Casting Solution A which contained .3 wt. percent glycerol ester lipid (triacetine). Line B represents the performance of the semi-permeable membranes cast from Casting Solution B which contained .6 wt. percent glycerol ester lipid (triacetin). Line C represents the performance of the semi-permeable membranes cast from Casting Solution C which contained no lipid. Each point on the lines represents performance at a given cure temperature indicated on the graph.

As can be seen from FIG. 5, the uncured semipermeable membranes containing lipids give from 1.15 to 2 times the flux of the uncured semi-permeable membrane with no lipids, at the same percent rejection. At 160 degree F. cure the semi-permeable membrane containing .6 wt. percent lipids gave 1.6 times the flux of the uncured semi-permeable membrane with no lipids, at the same percent rejection.

10 EXAMPLE 4 A casting solution was prepared having the following composition:

5 oz. 24.7 wt. percent cellulose acetate 9 oz. 44.6 wt. percent acetone 6 oz. 29.7 wt. percent formamide (methanamide) 5 ml. 1.0 wt. percent triactin (triacetate glycerol) The solid cellulose acetate was added to the other liquid ingredients to give a viscous liquid solution. This viscous casting solution was poured into the film casting apparatus, heretofore described and shown in FIG. 3 of the drawings, containing a blade with an opening of 7 mils. The casting apparatus was 6" long and it was hand drawn across a 6" x 4'6" glass plate in 50 seconds. The casting was allowed to air dry for another 10 seconds. Both the glass plate and the casting apparatus were at room temperature. No effort was made to increase the normal laboratory air circulation during the air dry time. The glass plate and casting were then immersed in a leaching bath prepared from a mixture of tap water and crushed ice. After 20 minutes the semi-permeable membrane casting 6 mils thick was separated from the glass plate. The osmotic skin side appeared glossy. From the semi-perme able membrane samples could be taken having air dry times varying from 10-60 seconds. Four 1" diameter samples were cut from the uncured semi-permeable membrane corresponding to about 10 seconds air dry time (from the very end of the casting) 20 seconds air dry time, 30 seconds air dry time (from the center of the casting) 40 seconds air dry time and 50 seconds air dry time.

The remaining semi-permeable membrane was immersed in a 160 degree F. water curing bath for 10 minutes. Three 1 diameter samples were cut from the semipermeable membrane corresponding to about 1'5, 20 and 30 second air dry time.

All seven of these semi-permeable membranes were mounted in standard stainless membrane cells heretofore described and shown in FIG. 4 of the drawings.

Tap water containing 100 p.p.m. dissolved solids was fed into the respective membrane cells against the osmotic skin sides of the membranes at p.s.i. The results are shown in FIG. 6, a graph of air dry time vs. purified water flux and percent rejection of dissolved solids in the tap water feed. Line D represents the performance of the uncured semi-permeable membranes. Line E represents the performance of the cured semi-permeable membranes.

As can be seen from FIG. 6, for this particular casting solution composition, excellent flux and rejection rates are achieved at air dry times between about 10-30 seconds. Other casting solution compositions such as those in Examples 1 and 2 indicate that a 60 second air dry time will produce excellent membranes and there is no reason to believe that air dry times of up to about 2 minutes will not produce good semi-permeable membranes.

In all of my membranes, after the acetone and formamide have been evaporated and leached out, the residue of lipid remaining in the polymeric film forming cellulosic material will be in the weight percent ratio of lipid to polymeric film forming cellulosic material of about 0.005 to 1 to about 0.09 to 1.

Although the examples have used the preferred ingredients in the casting solutions, it is felt that any semipermeable membrane or method of fabricating a semipermeable membrane wherein an amount of lipid effective to increase rejection of impurities in the feed is incorporated in the osmotic skin of the membrane, is within the scope of this invention.

I claim as my invention:

1. A process for the preparation of semi-permeable membranes which are adapted to permit the effusion of water and the rejection of dissolved impurities therein, which comprises:

(A) admixing a polymeric film forming cellulosic material, organic solvent, pore producing swelling ad- 1 l ditive and between 0.1 and about 4 wt. percent glycerol ester lipid to provide a casting solution,

(B) casting a film from said solution,

(C) drying said film to evaporate a portion of the solution, and

(D) leaching the film to form a semi-permeable osmotic skin membrane containing a residue of lipid effective to improve the rejection of impurities.

2. The process of claim 1 wherein the polymeric film forming cellulosic material is selected from the group consisting of cellulose esters and cellulose ethers, and the osmotic skin of the membrane has a plurality of foraminous passageways therethrough, the lipids being concentrated at the surface of the osmotic skin and in the passageways.

3. The process of claim 2 wherein the polymeric film forming material is selected from the group consisting of cellulose acetate, cellulose acetatebutyrate, cellulose propionate and ethyl cellulose and the film is dried in step (C) up to about 120 seconds.

4. The process of claim 2 wherein the organic solvent is selected from the group consisting of acetone, methyl ethyl ketone, ethyl alcohol and methyl alcohol.

5. The process of claim 2 wherein the swelling additive is selected from the group consisting of formamide, triethyl phosphate, tetrahydrofurfuryl phosphate and magnesium perchlorate.

6. The process of claim 2 wherein the organic solvent is acetone, the swelling additive is formamide and the membrane is leached in a water bath and thereafter cured at a temperature up to about 200 degrees F.

7. The process of claim 2 wherein the polymeric film forming cellulosic material is cellulose acetate and the film is air dried from 0 to about 60 seconds.

8. The process of claim 7 wherein the glycerol ester lipid has the formula:

12 wherein R, R and R are alkyl groups containing 1 to 20 carbons.

9. The process of claim 8 wherein the glycerol ester lipid is triacetin.

References Cited UNITED STATES PATENTS 2,925,352 2/1960 Lowe 106-196 X 3,567,809 3/1971 Ueno et al. 26441 3,646,179 2/1972 Stana 26441 OTHER REFERENCES U.S. Office of Saline Water, Investigation and Preparation of Polymer Films to Improve the Separation of Water and Salts. Research and Development Progress Report N o 69, by Monsanto, December 1962, pp. 5-18, 20, 21.

U.S. Office of Saline Water, The Mechanism of Desalination by Reverse Osmosis. Research and Development Report No. 117, By B. Kellin, Aerojet-General Corp. 1964, pp. 34; table 19.

Rose, Arthur and Elizabeth, The Condensed Chemical Dictionary, Seventh Edition, New York, Reinhold, 1966, p. 560.

Kirk-Othmer Encyclopedia of Chemical Technology, Second Completely Revised Edition, New York, Interscience, 1965, pp. 776-787.

Modern Plastics Encyclopedia, 1967, September 1966, vol. 44, No. 1A, pp. 422-429.

PHILIP E. ANDERSON, Primary Examiner U.S. Cl. X.R. 

